The invention relates to the control of an automatic transmission for a vehicle having a hybrid powertrain, in which both an internal combustion engine and a secondary power source, such as an electric motor, hydraulic motor, pressurized fluid accumulator or flywheel, provide power to the transmission input.
In hybrid electric vehicle applications, in which a secondary power source and engine supply torque to accelerate the vehicle, transmission gearshifts should occur at a lower vehicle speed than the speed at which they would occur if the engine alone were providing power. Producing earlier gearshifts improves fuel economy, but there is a need to determine the correct combination of operating conditions at which to produce the gearshifts so that they are stable and consistent. A dynamic method for determining the shift points is required because of the variability and limited energy storage capacity of the secondary torque device relative to that of the engine. For example, the energy storage capacity of an electric battery, an accumulator containing pressurized fluid, and inertia of a flywheel, are limited and vary with operating conditions of the vehicle and the driver's demands for power due to road conditions and terrain.
Some current production hybrid vehicles use automatic transmission control strategies, which maintain a constant engine speed versus vehicle speed relationship. The secondary torque source is used as a torque supplement to operate the engine in the best Brake Specific Fuel Consumption BSFC condition. BSFC is the fuel flow rate per unit power output. It measures how efficiently an engine is using the fuel supplied to produce work.
In a step-change type transmission that produces discrete torque ratios or gear states, the state changes are not transparent. A decision to change gears should be made on the basis of the ability of the powertrain to remain in the next gear for an acceptable period. Otherwise, engine lugging and shift busyness occur.
When a secondary power torque source is active during vehicle acceleration, the load on the engine is reduced. A gear shift strategy that produces gear shifts on the basis of engine torque and vehicle speed relies on an assumption that an upshift should occur based on the engine torque requirements. But in a hybrid powertrain, engine torque requirements are less than if the secondary power source were not assisting the engine to accelerate the vehicle. If an upshift occurs without accounting for the torque availability of the secondary power source, however, the engine torque requirements could vary substantially after the upshift begins due to the loss of torque from the secondary power source. In the event of a reduction in the magnitude of torque provided by the secondary torque source after an upshift begins, an immediate downshift will occur, which will degrade performance feel and reduce driver satisfaction.
To provide consistent shift points, while maximizing both fuel economy and performance, it is preferred that an electronic controller that commands transmission gear changes, allows early upshifts, provided there is sufficient energy available to the secondary power source. If the secondary torque source can provide torque for a sufficient period after the upshift is initiated, the transmission could upshift earlier without the risk of an immediate downshift. This would improve fuel economy. The early shift point can either be located on an additional upshift line that passes through hybrid shift points or it can be located on a conventional, normal gearshift line relating actual engine torque and vehicle speed. When hybrid assist is available, the engine torque will be reduced, allowing earlier upshifts. For conventional gearshift scheduling, the engine torque required will be the total actual torque requirement, i.e., the sum of the engine torque and torque produced by the secondary power source. This torque sum is driver demand output torque. When maximum performance is required, based on acceleration greater than a calibrateable value, shift scheduling should be based on the total actual torque requirement, and early upshifts are inhibited.